Hybrid location methods are known to exploit different localization techniques, such as satellite measures (using e.g. the GPS-Global Positioning System—navigation system) and cellular measures (e.g. based on the measure of the electromagnetic field). These known methods use suitable algorithms that, based upon two or more pluralities of measures acquired by the terminal, are intended to globally improve the performance of the positioning, in terms of accuracy, availability, and so on.
The estimation of the position of the mobile terminal is carried out either at the terminal itself or at a network server, also called mobile location center (MLC). In the latter case, the MLC receives the measures from the mobile terminal and, if necessary, uses additional data such as a network database (including data about the physical environment around the mobile terminal) and/or data received from reference GPS receivers.
U.S. Pat. No. 6,249,245 discloses a system combining GPS and cellular technology, wherein the mobile terminal calculates its position using GPS data and information sent by a cellular network. Information can include differential GPS error correction data. When the requisite number of GPS satellites are not in view of the GPS receiver, the system utilizes a GPS pseudosatellite signal that is generated by one or more base stations of the cellular network independent of the GPS. When the requisite number of GPS satellites is temporarily not in view, position is calculated using the cellular network infrastructure. In the alternative, cellular signals already transmitted from a base station are used to calculate a round trip delay and then a distance between the base station and the terminal, which replaces a missing GPS satellite signal.
WO 01/86315 discloses a system, wherein a mobile unit compiles a set of positioning data based on the signals received from GPS satellites and from a cellular communication network. The positioning data are then transferred to a calculation center where the most precise location calculation possible is performed.
Applicant has noted that these known systems have the inherent shortcoming of assuming that the cellular and/or satellite measures are available to the base station.
Other GPS solutions provide for an “assisted GPS”, wherein the network server sends suitable data for improving the receiver performances.
For example, U.S. Pat. No. 6,389,291 discloses a GPS system that can operate in different modes, including a standalone mode, wherein a mobile communications device computes its position; an autonomous mode, where the mobile communications device transmits the computed position to another component of the communications network; a network aided mode, wherein the network aids the mobile communications device; and a network based mode. In this system, the selective switching of the GPS receiver is performed either automatically or manually at the wireless communications device in either local or remote control.
Applicant has noted that this system has the inherent shortcoming that it does not take into account the terminal resources which thus cannot be efficiently managed. For example, this system may decide to use an autonomous or standalone mode (i.e. where the mobile device computes its position) and thus decide to wait until a position calculation has been done; but this choice could be not optimal when the battery level is too low (and thus does not enable data to be acquired within the provided acquisition time). Furthermore, the choice at the terminal's end does not take into account the calculation power existing at the server. In fact, Applicant has observed that the performances of a GPS/AGS (Assisted Global Positioning System) mainly do not depend upon the operation mode (i.e. with or without assistance data transmission), but they mainly depend upon the local conditions of the mobile terminal and are generally unpredictable.
In general, the only choice of the operation mode is not sufficient to have an efficient localization procedure and does not ensure the optimal trade-off between requirement compliance and resource management. For example, for preset accuracy requirements, it may be not efficient to wait for the terminal to calculate the position, when the network server has a suitable algorithm able to process a subset of measures that can be acquired in a shorter time.
US 2002/0019698 describes a solution wherein a positioning method selecting device automatically determines the best possible positioning method available for use by the terminal's applications, based on requirements specifying the quality of service.
US 2003/0045303 describes a system wherein the terminal receives assistant information from a positioning server and measures signals based onto the received information; in case the positioning information collected is sufficient, it displays the location computed by the positioning server; otherwise it displays a corresponding message.
EP-A-1 418 439 discloses a portable telephone aimed at reducing power consumption and shortening positioning time, by selecting a method for determining the location according to the distance from an objective point or a reception level of a downlink signal. The telephone automatically switches a positioning method according to the distance between its location and the objective point. In detail, the portable telephone receives the location of the base station when in a distant place from the objective point, regards it as the location of the telephone and exerts control to gradually increase the number of times the GPS positioning is selected as it come closer to the objecting point.
US-A-2003/0144042 discloses a method wherein the rate at which position information is transmitted may be adjusted based upon the battery strength and/or whether the mobile station is operating in the emergency mode. The mobile station determines whether the battery is low and transmits this information to the network. The mobile station may also transmit whether it is currently operating in an emergency mode. Based on this information, the network may adjust the rate.